1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system which has a function of setting the degree of enriching the air-fuel ratio of a mixture supplied to the engine at the restart of fuel supply to the engine immediately after termination of fuel cut-off, according to the fuel cut-off period.
2. Prior Art
When cutting-off of fuel supply to an internal combustion engine (hereinafter referred to as "fuel cut-off") is carried out, the amount of an oxygen component (O2) in exhaust gases from the engine relatively increases, and the increased amount of oxygen is absorbed by an exhaust gas purifying device within the exhaust pipe so that the interior of the purifying device is brought into an oxidizing atmosphere. Consequently, nitrogen oxides (NOx) emitted from the engine at the restart of fuel supply following fuel cut-off are not well reduced by the exhaust gas purifying device, resulting in an increased emission amount of NOx. To mitigate the oxidizing atmosphere into which the interior of the exhaust gas purifying device is brought, it has been proposed to measure the fuel cut-off period, set an initial value of an increment for the fuel injection amount after fuel cut-off according to the fuel cut-off period, and then progressively decrease the fuel injection amount with the lapse of time, by Japanese Patent Publication (Kokoku) No. 63-27533.
According to this prior art technique, however, in the case where the engine is repeatedly accelerated and decelerated alternately so that fuel cut-off is carried out and then fuel supply is restarted, followed by fuel cut-off, the measured fuel cut-off period is reset to zero at the restart of fuel supply. Consequently, at the restart of fuel supply, an insufficient increment is applied to the fuel injection amount, and accordingly O2 absorbed by a catalyst of the exhaust gas purifying device cannot be desorbed to a sufficient degree to reduce NOx in exhaust gases, which can result in degraded exhaust gas emission characteristics.